Recently, an antenna which is called a shark fin antenna has been developed. The height of the antenna is reduced, for example, to about 70 mm. On the other hand, the length of the antenna is increased in order to ensure the gain. Hence, the antenna base is larger as compared to a prior art one. From the viewpoints of the weight reduction and the cost reduction, therefore, a structure in which the antenna base is made of a resin is proposed. Patent Literature 1 below discloses a structure in which a metal-made base is assembled to a resin-made base from an inside of an antenna. In this structure, the size of the metal-made base is set in accordance with a size of the mounted antenna and the necessity/unnecessity of grounding depending on a antenna system, and the base of an antenna in which grounding is unnecessary is made of a resin, whereby the metal-made base can be miniaturized as far as possible.